The aim of automation technology is for machines or systems to operate completely autonomously and independently of people. The degree of automation is higher, the better this aim is achieved. For this purpose, all levels, from a central controller (the so-called PLC) to the motor branch circuits or motor controllers at the field level, must communicate with one another.
In this case, motor branch circuits protect and control the motor independently of the superordinate central controller. The motor remains completely protected and controllable even if the PLC fails or in the event of a fault in communication with this PLC.
Motor branch circuits known nowadays are already modular and are therefore in the form of flexible systems. In this respect, a basic unit as a basic module already performs all protective and control functions needed to operate the motor branch circuit. If necessary, this basic module can be supplemented with expansion modules having additional functions via its system interfaces. For example, the type and number of binary or analog inputs and outputs can be gradually increased. Alternatively, an additional current or voltage detection module can be used to monitor power-related measurement variables (for example for energy management), etc. However, so that these functions can also be used, the expansion modules must communicate and continuously interchange data with the basic module.
WO 91/14324 A discloses a method and a communication system for serially transmitting data. In this case, data are transmitted using one or more input and/or output modules under the control of at least one master unit. Said document describes that the input/output modules can be connected to the master unit via one or more slave units and a serial data transmission system.
DE 102 24 311 A1 has already described a method for serial communication between a basic module and a number of expansion modules, which method could thus be used in a motor branch circuit. In the case of the bidirectional communication method described here, data are shifted and interchanged between the basic module and the subsequent expansion modules, in the sense of a serial shift register, in such a manner that, in addition to easily recognizing the expansion modules, data can also be transmitted with little technical effort.
However, this method has the disadvantage that, as a result of the serial communication through all modules, communication between the basic module and the expansion modules is relatively slow since all expansion modules must heed the data sent by the basic module.